A heat-developing process in which development is carried out by heating is a known technique, examples of which include those for obtaining black-and-white and color images. Transfer-type heat-developable light-sensitive materials, in which an image obtained by heat development is transferred onto an image receiving layer, are also well known.
The transfer-type heat-developable light-sensitive material is classified into two: one comprising a light-sensitive material having an image-receiving layer capable of receiving silver or dyes, and the other comprised of a light-sensitive material alone, which is used together with a separate image-receiving member having an image-receiving layer capable of receiving silver or dyes.
In an image forming method which uses the above heat-developable light-sensitive material, if desired to obtain a more stable or clearer image, a developed image is preferably transferred from the light-sensitive material onto an image-receiving material. In this instance, preferably the heat-developable light-sensitive material and the image-receiving material are formed by coating appropriate light-sensitive layers and image-receiving layer, respectively, on different supports. The heat-developable light-sensitive material is imagewise exposed, heated for developing to form or release a diffusible dye image or a silver image in the light-sensitive layer thereof, and then is superposed upon the image-receiving material and, if necessary, pressed and heated for forming a diffused dye image or a silver image; or alternatively, the imagewise exposed heat-developable light-sensitive material is superposed upon the image-receiving material, and then subjected to heat/pressure treatment for developing simultaneously with transfer of a diffusible dye or silver image onto the image-receiving layer, whereby a clear and well preservable image can be obtained.
One problem of such heat-developing methods is expensiveness due to the need of two supports for the image formation. When a conventional support such as polyethylene terephthalate (PET) support is used, the support, when heated, tends to cause an image distortion due to lowering of its stiffness or heat contraction or, when transported at the time of heat development, tends to cause a jamming trouble. For this reason, an attempt has hitherto been made to use paper as the support for heat-developable light-sensitive materials as described in JP O.P.I. No. 248148/1989.
The use of paper as the support for heat-developable light-sensitive materials has some major problems.
A first problem is that a paper support is hygroscopic, and the hygroscopicity of paper largely varies depending on the storage conditions, particularly humidity, of heat-developable light-sensitive materials to thus not only change the photographic characteristics but also largely affect the curl characteristic of the heat-developable light-sensitive material. As a result, the heat-developable light-sensitive material takes various curl forms combined with other factors such as the wind curl formed when stored in roll form, and tends to generate problems of causing jamming trouble of the light-sensitive material when transported through an exposure device, heat-developing device or transfer device; a failure in superposing a heat-developable light-sensitive material upon an image-receiving material, allowing their transport to be in a crooked state; and a registering failure when they are to be registered correctly.
A second problem is that, when heat developing is made without supply of moisture from the outside, the paper support is left extremely dried, which can cause the heat-developable light-sensitive material to be badly curled while staying in the heat developing device. The heat-developable light-sensitive material is normally designed so as to cause little or no curl and so as to obtain a proper curl balance at normal temperature. Under this condition, paper retains usually a water content of about 4% to 8%. Where most of its water content is evaporated by heating, the paper support becomes badly curled to tend to cause an uneven developing or a failure in contacting the heat-developable light-sensitive material with the image-receiving material inside the heat developing device.
A third problem is that, when heat developing is made with a small amount of water supplied from the outside, the paper support absorbs a considerable amount of moisture, so that the stiffness of the heat-developable light-sensitive material is largely lowered to cause a jamming trouble or break of the light-sensitive material.
A fourth problem is that some of the chemicals added to the heat-developable light-sensitive material are diffused into its paper support during coating its photographic layer, during its storage or during its heat-developing time. Such the problem occurs particularly when the additives used are relatively hydrophilic and relatively low-molecular compounds, and further is liable to occur when the photographic layer is formed by coating an aqueous coating liquid or when the light-sensitive material is stored under high humidity conditions (e.g., at a relative humidity of not less than 60%). This problem tends to occur also when its coating is made in the presence of a relatively high hydrophilic solvent or a thermal solvent, and when heat development is carried out in the presence of a relatively high hydrophilic solvent or a thermal solvent.
As a result, necessary additives that should be present at the time of heat developing become unable to be sufficiently present, thus making it difficult to obtain satisfactory photographic characteristics.
Especially, the above fourth problem significantly affects photographic characteristics, so that it is essential, particularly for a heat-developable light-sensitive material containing hydrophilic photographic additives, to take measures for preventing diffusible additives from permeating into the paper support. Accordingly, in the heat development of a heat-developable light-sensitive material having a paper support, there is a proposal for providing a hydrophobic polymer layer between the paper support and the light-sensitive layer.
JP O.P.I. No. 38934/1988 discloses the use of a low-hygroscopic support of which the light-sensitive layer-provided side's cup hygroscopicity specified in JIS-P-8140 is not more than 3 g/m.sup.2, and also discloses that a hydrophobic resin is coated preferably in an amount of 1 to 30 g per m.sup.2 of the paper support of a light-sensitive material to thereby prevent transfer unevenness of a formed image.
JP O.P.I. No. 314539/1988 discloses a heat-developable light-sensitive material having a low-hygroscopic paper support of which the light-sensitive layer side's cup hygroscopicity specified in JIS-P-8140 is not more than 1 g/m.sup.2, the paper support having thereon a hydrocarbon resin-containing layer to reduce transfer unevenness and prevent the degradation of sharpness. To be concrete, a hydrocarbon resin-containing layer are provided in an amount of 2 g to 20 g per m.sup.2 on the paper support and on the layer is coated a heat-developable light-sensitive layer to form a heat-developable light-sensitive material.
Where a heat-developable light-sensitive material is prepared using such a support, the use of the hydrophobic resin, because of its water-impermeability, is a useful solution of the above fourth problem. However, it has been found that this alone is not effective to solve the first to third problems and, where on the hydrophobic resin layer is provided hydrophilic binder-containing light-sensitive layers or other photographic layers, the resin layer may rather increase the deterioration of the curl of the heat-developable light-sensitive material at room temperature or at the time of heat developing unless the hydrophilic binder's amount is confined within certain limits.
Also, it has been found that, when a resin as described in the above O.P.I. publications is used, the diffusible dye or dye precursor resulted from heat development is relatively prone to permeate into the resin layer.
JP O.P.I. No. 179937/1989 discloses a light-sensitive material having hollow spherical thermoplastic resin particles-and binder-containing layer between its paper support and its light-sensitive layer, and the light-sensitive material is preferably subjected to heat development. The above binder is preferably a hydrophobic binder, and preferably used in an amount of 5 g to 20 g per m.sup.2 of the light-sensitive material. It discloses that by using a heat-developable light-sensitive material having such the paper support there can be obtained a high-resolution transfer image free of transfer unevenness. However, in a heat-developable light-sensitive material having a hydrophilic binder on its light-sensitive layer side, even the technique in the above disclosure has not attained any effective solution means to the curl at room temperature or at the time of heat development.
JP O.P.I. No. 248148/1989 discloses a light-sensitive material comprising a castcoat paper support having thereon a hydrophobic polymer-containing layer, on which is provided a light-sensitive layer. The light-sensitive material is preferably used as a heat-developable light-sensitive material and capable of forming an image with still further reduced transfer unevenness. However, even when such a heat-developable light-sensitive material is used, similarly to the aforementioned prior art, the curl at room temperature or at the time of heat development is not always sufficiently controllable.
Further, in the techniques described in the above patent publications, the hydrophobic polymer layer is provided between the paper support and the hydrophilic binder-containing light-sensitive layer, but in this instance it is necessary to secure the adhesion between the paper support and the hydrophobic polymer layer and between the hydrophobic polymer layer and the light-sensitive layer. Particularly in a method in which a heat-developable light-sensitive material and/or an image-receiving material are heated to be molten to form an image or in a method in which immediately before heat development water or other relatively high hydrophilic solvent is supplied to a heat-developable light-sensitive material and/or an image-receiving material to thereafter make dye transfer, there occurs the disadvantage that the adhesion strength between the light-sensitive layer and the paper support tends to be lowered, resulting in peeling of the light-sensitive layer.